Since we now have the MintUpgrade which apparently allowed users to upgrade from Mint 18 to 19 does that mean we don't actually have to re-install the operating system now? Will this software mintupgrade work for v19 users when Mint 20 gets released? Could someone clarify this for me?

Yes, you can upgrade in place between minor releases (19 to 19.1) using Update Manager and between major releases (19.3 to 20) using the mintupgrade tool. The tool is tailored for each major release.

Upgrading in place between minor releases is mostly trivial because only the Linux Mint software gets upgraded. Upgrading in place between major releases is a bit more involved because all software on your system gets upgraded. Things like added repositories or PPAs can break and require edits to make them work. The upgrade instructions have users make a system snapshot and a backup. So should something go wrong they can roll back the upgrade and restore their previous version of the operating system. Their personal files are safe in the backup they made.

I disagree that a clean install is always the best option. It depends on the user.